A novel mechanism linking corticotrophin-releasing factor to ventral tegmental area dopamine neuron firing

Corticotrophin-releasing factor (CRF) is a 41 amino-acid peptide that plays a central role in a variety of stress-related conditions (Van Den Eede et al. 2005). Especially, CRF is widely recognized as a key factor in the coordination of neuroendocrine, autonomic and behavioural responses to stress in mammalian species. Numerous studies have also indicated the involvement of the CRF system in various diseases including Alzheimer's disease, rheumatoid arthritis and depression (Van Den Eede et al. 2005). CRF exerts its functions through its two distinct receptor subtypes, CRF-R1 and CRF-R2, which are class B G-protein-coupled receptors. In the human brain, CRF-R1 and CRF-R2 are widely distributed in areas such as the cortex, pituitary, hippocampus, amygdala, cerebellum and thalamus (Van Den Eede et al. 2005). Growing evidence supports the idea that CRF receptor activation modulates dopamine release and metabolism in several brain regions. Numerous investigations have suggested that the change in synaptic dopamine in the nucleus accumbens (NAc) is involved in brain reward mechanisms, drug addiction and other behaviours. Since dopamine is released in the NAc from neurons projecting from the ventral tegmental area (VTA) of the midbrain (Hyman et al. 2006), various studies of addiction have focused on VTA dopamine content. In one such study, CRF was shown to potentiate NMDA receptor currents in dopaminergic neurons of the VTA (Ungless et al. 2003). Other data also hint that CRF might be involved in regulating VTA activity. However, direct evidence regarding the cellular effect of CRF on the excitability of VTA dopamine neurons still remains largely elusive.

In a recent study in The Journal of Physiology, the link between CRF and the dopamine system was further validated by the identification of a novel signalling mechanism underlying the CRF-induced increase of dopaminergic neuronal firing in the VTA (Wanat et al. 2008). In this study, the authors first used biocytin to label recorded neurons to examine the correlation of I_H with VTA dopaminergic neurons. Almost all (98%) I_H-expressing neurons were demonstrated to be dopaminergic. Such a high correlation validates the usage of I_H as an indicator of dopaminergic neurons in the following experiments.

The authors found that bath application of CRF enhances the firing rate of putative dopaminergic neurons in a dose-dependent manner. Then the authors employed an arsenal of activators and inhibitors of CRF receptors and second messenger signalling pathways, with the aid of transgenic mice, to dissect the mechanism underlying CRF's effect on neuronal excitability. Both CRF-R1 and CRF-R2 exist in the VTA and could mediate the effect of CRF. However, only a specific CRF-R1 antagonist and a general CRF receptor antagonist were found to impair the CRF-induced increase of dopamine neuron firing. A CRF-R2 antagonist had no effect. Consistent with this finding, an agonist of CRF-R1 dramatically enhanced the firing rate of VTA neurons, whereas an agonist of CRF-R2 had no effect. This selectivity was confirmed in knockout mice, where the authors found that CRF-R1, but not CRF-R2, was necessary for the physiological effects of CRF on VTA neurons.

Although CRF receptors typically activate the cAMP–PKA signal transduction pathway, the authors found that CRF could still increase VTA firing when PKA was pharmacologically inhibited. Surprisingly, the authors found that blocking the phospholipase C (PLC)–protein kinase C (PKC) signalling pathway abolished the effect of CRF on neuronal spiking. Intrigued by the reduction of the after-hyperpolarization potential induced by CRF, the authors next investigated if any ion channel plays a role in mediating CRF's effect. Those of particular interest were known to encode calcium-activated potassium currents (I_K(Ca)), inwardly rectifying potassium currents (I_Kir), A-type potassium currents (I_A) and hyperpolarization-activated cation currents (I_H), each of which has been shown to regulate the frequency of neuronal firing. The authors examined these currents and found that the increase in VTA dopaminergic neuronal firing rate initiated by CRF is not sensitive to inhibitors of these currents, except for I_H. When I_H was blocked by the specific channel antagonist ZD-7288, the authors found that CRF was no longer able to enhance neuronal firing. To tie everything together, the authors also showed that CRF treatment of VTA dopaminergic neurons potentiates I_H and that this potentiaion is dependent on PKC, though the exact mechanism underlying this effect is not clear yet. It is possible that phosphorylation of the channel or accessory proteins by PKC causes channel potentiaion, but further experiments and evidence are required to demonstrate this definitively.

The authors therefore demonstrated a novel link between CRF and VTA dopaminergic neuronal firing through a PLC–PKC and I_H-dependent pathway. Elucidating the mechanism underlying the effects of CRF on dopamine neurons provides valuable clues toward understanding how stress imposes its influence on multiple types of behaviour. Besides its well-known role in producing anxiety-like symptoms (mediated by the activation of the hypothalamus–pituitary–adrenal axis), stress is thought to be associated with a variety of dopamine-dependent behaviours, such as locomotor activity and reward-related behaviours. In particular, stress has long been recognized as an important factor that interacts with addictive behaviours in a profound manner. For example, exposure to stressful stimuli can initiate drug taking and can reinstate drug seeking and taking even after prolonged periods of abstinence (Goeders, 2003). Relevant to the present study, direct administration of CRF into the brain can sensitize the behavioural effects of certain addictive drugs, and exposure to addictive drugs can exaggerate stress-induced responses. These complex interactions may lead stress responses and addictive behaviours to strengthen each other in a devastating fashion.

However, the underlying neural mechanism that links stress and addiction is largely unknown. Addiction is thought to be mediated by an integrated brain circuit, the key structures of which include areas like the VTA, the NAc, the amygdala and the prefrontal cortex. Synaptic plasticity occurring in the dopaminergic neurons of the VTA has been hypothesized to play a key role in mediating initiation and maintenance of addictive behaviours (Hyman et al. 2006). To understand how stress interacts with addiction, one necessary step would be to test if stress can induce synaptic plasticity in the brain circuit involved in drug addiction. Indeed, one pilot study focusing on the effects of stress on VTA dopamine neurons has shown that acute exposure to stressful events can enhance synaptic strength in those neurons (Saal et al. 2003). The effects of stress on synaptic strength are reminiscent of those induced by acute exposure to different classes of addictive drugs, implying a common cellular mechanism might be shared (Saal et al. 2003). Given that stress stimulates CRF release in the VTA, the study conducted by Wanat et al. (2008), by identifying the effect of CRF on the I_H current in dopaminergic neurons, reveals a candidate cellular mechanism that contributes to stress-induced synaptic plasticity. Since both stress and addictive drugs induce synaptic plasticity in dopaminergic VTA neurons, one interesting question that could now be asked is whether exposure to addictive drugs has similar effects on I_H. Regardless, in conjunction with other data suggesting that stress stimulates CRF release in the VTA and induces dopamine release in the brain regions mediating the development of addictive behaviours, it is very likely that stress-induced activation of the VTA dopamine system has great functional significance in promoting addictive behaviours. Thus, these novel findings made by Wanat et al. (2008) are of great importance and are expected to stimulate more interest in investigating the interactions of stress with addiction, as well as with other psychiatric conditions such as anxiety, depression and schizophrenia.